Answer:
mew
a) Have a Pokemon with Fly/Teleport in your party
b) Save the game
c) There is a Long Range Trainer in the grass Left of the Nugget Bridge, when you step into his sight (you can also use the Gambler near the Underground Path.), quickly press Start
d) Now Fly/Teleport to Cerulean City PokeCenter.
e) If you haven't beaten all the Trainers in Cerulean Gym, fight the first trainer; If you did beat the Gym already, fight the Youngster with the level 17 Slowpoke.
f) Fly/Teleport to the Center nearest to where you started the Glitch, when the Menu pops up, press B then Fight and Catch the Mew that appears.
You can use this Glitch to get 2 Mews before fighting Misty. You can also use it to get other Pokemon by fighting other Trainers.